Latent curing agents are solid low molecular weight chemicals that are dispersed in monomers, oligomers and polymers or mixtures thereof. Those chemicals are practically inactive at ambient temperature and become active at elevated temperatures. When activated, the dispersed solid particles of latent curing agent melt and react or initiate polymerization with polymers to form a cross-linked polymeric network.
A thermoset polymer is a polymer that doesn't melt when heated. The typical latent curing agents in thermosets, especially in epoxy systems, are solid fine powders that are dissolved in the epoxy matrix only above critical temperatures.
Epoxy based thermosets are cost effective matrices known for their low viscosity, excellent chemical and thermal properties, cost effectiveness, excellent adhesion to metals, ceramics and plastics, ease of application, low toxicity, wide spectrum of resins, diluents, modifiers and curing agents. The most popular epoxy resins for high performance applications are Diglycidyl ethers of bisphenol A (hereinafter ‘DGEBA’), epoxy phenol Novolacs (hereinafter ‘EPN’), epoxy cresol Novolacs (hereinafter ‘ECN’), Diglycidyl ethers of bisphenol F (hereinafter ‘DGEBF’), and bisphenol A based Novolacs.
Epoxy is the major building block of most high-end adhesives, legend marking inks, solder masks and hole plugging pastes. The most common latent curing agent for epoxy resins are urea derivatives, imidazoles, modified amines, and dicyandiamide (hereinafter ‘DICY’), which is a commercially available ingredient in high-end electronic applications, such as adhesives, PCBs manufacturing, solder masks and hole plugging inks. DICY is characterized by a significant latency (pot life) and outstanding final properties of the cured matrix. It is activated at temperatures in the range of 160-170° C. without a catalyst and in the range of 120 to 150° C. with catalyst. The catalyst usually has negative impact on a pot life of the mixed formula.
Ink jet technology enables a very precise digital printing and provides for relatively small dots of liquid or molten material. Digital printing of high performance polymers adapted for marking, protection, coating, encapsulation and definition in electronic manufacturing has become a reality in recent years. The aforementioned ink is characterized by a low viscosity, less than 15 Cp at jetting temperature. When solid particles are used, their maximal size must be sub-micron, preferably less than 700 nm. These particles are usually selected from fillers, pigments, curing agents, rheology modifiers and matting agents. Thermoset inkjet inks, and particularly epoxy-containing ink-jet ink are latent high performance latent resins and particularly epoxy-containing particles of latent curing agent such DICY. Nevertheless, the finest commercially available DICY powders are characterized by an average particle size of 4 to 6 micron, thus their use in ink-jet application is avoided. A sub-micron latent curing agent is hence a critical component for high performance digital printing inks and its commercial availability ass a long felt need.